Thread: static class members

My compiler gives me this error message when I try to build the project --

Code:

1>Aisle.obj : error LNK2001: unresolved external symbol "private: static int Aisle::numberOfAisles" (?numberOfAisles@Aisle@@0HA)
1>C:\Users\Sterling\Documents\Visual Studio 2008\Projects\Gas Station\Debug\Gas Station.exe : fatal error LNK1120: 1 unresolved externals

The code for the Aisle class is

Code:

class Aisle
{
public:
	Aisle(void);
	~Aisle(void);

	void setNumber(int);
	int getNumber();
	static int getNumberOfAisles();
private:
	int number;
	static int numberOfAisles;
};

Code:

#include "Aisle.h"

Aisle::Aisle(void)
{}

Aisle::~Aisle(void)
{}

void Aisle::setNumber(int aNum) {
	number = aNum;
}

int Aisle::getNumber() {
	return number;
}

int Aisle::getNumberOfAisles() {
	return numberOfAisles;
}

What am I doing wrong with numberOfAisles? I declare it as a private static int, then make a static function to return it. I don't see why there is a problem.

You have to define it. Since it is static, there is only one instance of it no matter how many class instances are created. A normal class member variable is constructed when the class object is constructed, but where is the single static variable constructed? You have to define it (and initialize it if desired) for the compiler.

It's easy to do, just add this to your cpp file:

Code:

int Aisle::numberOfAisles = 0; // or whatever initialization you want

Whenever I create a new instance of an Aisle, will

int Aisle::numberOfAisles = 0;

set numberOfAisles to zero each time?